The use of LNG (Liquefied Natural Gas) as fuel for marine applications is increasing since it is an efficient way of cutting emissions. Within the next few decades, natural gas (NG) is expected to become the world's fastest growing major energy source. The driving forces behind this development are the depleting known oil reserves, increasing environmental care and the continuous tightening of emission restrictions. All major emissions can be significantly reduced to truly form an environmentally sound solution; the reduction in CO2, for example, is difficult to achieve with known oil-based fuels. NG contains (e.g., consists of) methane (CH4) with minor concentrations of heavier hydrocarbons such as ethane and propane.
In normal ambient conditions NG is a gas, but it can be liquefied by cooling it down to −162° C. In liquid form the specific volume is reduced significantly, which allows a reasonable size of storage tanks relative to energy content. The burning process of NG is clean. Its high hydrogen-to-coal ratio (the highest among the fossil fuels) means lower CO2 emissions compared with oil-based fuels. When NG is liquefied, all sulphur is removed, which means zero SOX emissions. The clean burning properties of NG also significantly reduce NOX and particle emissions compared with oil-based fuels. LNG is not only an environmentally sound solution, but also economically interesting at today's oil prices.
A feasible way of storing NG in ships is in liquid form. In existing ship installations, LNG is stored in cylindrical, double-walled, insulated stainless steel tanks. The tank pressure is defined by specifications of the engines burning the gas and can be less than 5 bar. A higher (e.g., 9 bar) tank design pressure can be selected due to the natural boil-off phenomenon.
FIG. 1 discloses schematically a known LNG installation for a ship 20. LNG is stored in a cylindrically shaped pressurized storage tank 1. The tank can be a stainless steel inner shell, which is designed for an internal pressure, and an outer shell that acts as a secondary barrier. The outer shell can be made of either stainless steel or carbon steel. The tank is insulated with perlite/vacuum.
Reference numeral 24 denotes a bunkering station from which LNG is led to the tank 1 via insulated pipes. The tank room 4 is a stainless steel barrier welded to the outer vessel of the tank 1. The tank room acts as a barrier that prevents damage to the external compartments, and facilitates quick ventilation of the evaporated gas. The LNG from the tank is evaporated and fed via a gas valve unit 21 to the engines. The main engine generators are denoted by reference numeral 22 and the switch gear by reference numeral 23. FIG. 1 is shows an exemplary schematic arrangement for an LNG installation and, therefore, there is no detailed explanation of a control system, thrusters, propulsion units or other implementation features included in ships.
NG is a safe fuel when proper precautions are taken.
In a liquid state LNG is not explosive, nor is it corrosive or toxic. Thus, possible spillages will not cause any lasting contamination, as the liquid will boil to gas. The low temperature, however, is an issue when considering normal ship steel, but this can be avoided by using appropriate materials in LNG systems.
Gaseous NG is lighter than air, which means that in case of a leakage, the gas will disperse upwards and not build up in the ship's bilge. The ignition temperature of NG is relatively high (600° C.) compared with diesel oil (250° C.), and NG is flammable only within a small concentration range from 5% to 15% of air.
The gas fuel system of a ship includes liquid storage tanks, a vaporiser, a gas valve unit, piping and a bunkering system.
The storage tank and associated valves and piping should be located in a space designed to act as a secondary barrier in case of liquid or compressed gas leakage. The material of the bulkheads of this space should have the same design temperature as the gas tank, and the space should be designed to withstand the maximum pressure build-up or, alternatively, pressure relief venting to a safe location (mast) may be provided. The space should be capable of containing a leakage and be thermally isolated so that the surrounding hull is not exposed to unacceptable cooling in case of a leakage of liquid or compressed gas.
The natural gas is delivered to the engines as a gas but stored as a liquid. A ‘tank room’ is associated with a storage tank and contains the equipment for converting the liquid into a gas for safe delivery to the engines. The tank room is also considered a ‘secondary barrier’ since the liquid pipes are inside it.
The piping between the LNG tank and the tank room is double-walled and pipes can be arranged to go through the outer shell of the LNG tank and pass into the space between the inner and outer shells of the LNG tank before they are connected to the inner shell, such as by welding. This known arrangement is functional as such but it involves the outer shell of the LNG tank being made of stainless steel since all connections to the inner shell should be inside a stainless steel cover.